The present invention relates to a cover for a hard disk drive which dampens rocking motion of the spindle motor, reduces the motion of the recording head relative to the disk, decreases the transmission of mechanical vibration from the spindle motor to the top cover and reduces acoustical emissions.
Hard disk drives are used in personal computer applications for the high volume storage of data. These drives contain a disk assembly and a head arrangement for transferring data to and from tracks disposed concentrically on one or more disk surfaces. The disks are mounted to a bearing spindle hub which is rotatable around an inner stationary shaft. A motor is typically mounted within or beneath the hub and rotates the disks and hub.
Three modes of vibration may occur in a spinning disk and hub assembly. The first mode of vibration for the disk and hub is in a radial direction relative to the spindle shaft. The second mode of vibration for the disk and hub is in an axial direction relative to the spindle shaft. The third mode of vibration is a rocking displacement of the disk and hub relative to the spindle shaft. Consequently, vibrational energy transmitted to the spindle-disk assembly may cause servo systems errors and track misregistration, thereby decreasing drive performance.
The vibrations occur for several reasons. A first reason is a result of spindle generated vibrations from ball bearing defects as the bearing spins during operation. A ball bearing is not perfectly spherical and generally contains some defect such as a flat spot, crevice or the like. Consequently, the movement of the spindle as the bearing passes each defect produces an excitation which generates vibration in a spindle. With several bearings defect frequencies associated with each spindle speed, a multitude of ball bearing excitation frequencies will produce vibrations in any given spindle design. A second reason is a result of environmental vibrations or shock. Sources of environmental induced vibrations include but are not limited to physical jarring of the disk drive installed in a computer, or any movement to a computer. A third reason is a result of vertical diaphragm vibrations of the head-disk assembly transferred to the spindle-disk assembly. The diaphragm mode vibrations are vertical drum-like deformations of the top cover and bottom base plate of a head disk assembly enclosure.
In addition to the foregoing described vibrations, every given spindle structure inherently has an upper and lower rocking mode as a result of its design in combination with manufacturing tolerances among its component parts, including the structural stiffness of the disk drive housing. Thus, any given spindle will exhibit specific upper and lower natural rocking resonances, which resonance or frequency will change depending upon the number of disks supported by the spindle, as well as the rotational speed of the disks. Moreover, the rocking resonance can also be excited by the bearing defect frequency (e.g., the number of cycles or number of regular passes of a defected bearing portion in a given amount of time) if the frequency of the natural rocking mode and the bearing defect frequency are close to each other. Just like every manufactured spindle is unique, every bearing design has a unique set of bearing defect frequencies based on the geometry of the bearing and the speed at which the bearings spin. As a result, an abnormally large radial vibration of the spindle will be produced when the bearing defect frequency is the same as or close to the natural rocking mode or frequency of the spindle and disk combination, i.e., the upper and lower rocking modes.
The rocking resonance of the spindle and the overall vibration induced into the spindle-disk combination is further affected by the stiffness of the disk drive housing. Typically, the spindle is positioned between the base plate and cover of the disk drive housing. The stiffness and damping of the base plate and cover can alter and/or dampen the natural rocking resonance of the spindle and the overall vibration of the spindle-disk combination. Disk drives with rigid shafts mounted to rigid housings offer minimum damping to attenuate the effects of spindle rocking mode and vertical diaphragm mode resonances caused by environmental shocks and vibrations, and spindle generated excitations from bearing defects, nor does such rigidity shift or alter the natural rocking resonance away from the bearing defect frequency.
The spindle-disk assembly structure also has a significant effect on the amplitude of vibrations resulting from spindle rocking mode and vertical diaphragm mode resonant frequencies. The amplitude of these vibrations is directly associated with drive performance. Undamped structures exhibit vibrations of higher amplitude at their resonant frequencies, compared to equivalent structures which contain damping, and thus are more likely to effect servo positioning and track registration. Consequently, for a given vibrational input from a source such as spindle bearing defects, an undamped and rigid disk drive housing containing the spindle-disk assembly will produce larger amplitude vibrations in the spindle at its resonant frequency than an equivalent disk drive housing containing damping.
Interaction between the rocking mode frequency and the bearing defect frequency creates a large non repetitive run out (e.g., movement of the spindle-disk in the radial direction) in the spindle causing large amounts of repetitive run out if it occurs during servo write, and large amounts of non repetitive run out if it occurs when the drive is in operation. These results will cause increased position error between the recording head and the data tracks resulting in reduced track following capability. This, in turn, can lead to servo system failure resulting in increased track misregistration, perhaps to the point of producing a non-functional drive, and/or, at a minimum generation of acoustic noise.
Where a common spindle structure is utilized for drives containing, for example, from one to four disks, and operated at two different speeds, there are 16 upper and lower dynamic rocking mode frequencies, and two distinct sets of defect frequencies, creating a large number of interactions between the rocking mode and bearing defect frequencies which need to be avoided. With different spindle vendors using different bearings with different geometries, there exists numerous additional opportunities for the bearing defect frequency and rocking mode frequency interactions to occur. Thus, given the variety of bearing geometries and spindle designs available, and the need to keep the structural stiffliess of the drive housing high for external shock and vibration considerations, it is virtually impossible to avoid all frequency interactions with a single common spindle design. Thus, disk drive manufacturers are required to inventory a large variety of different spindles, to accommodate multiple disk drive platforms and performance standards in their overall product line.
It would be useful to find a way to modify the structural stiffness of a spindle to avoid or dampen generating vibrations due to natural rocking resonance and/or interactions of bearing defect frequencies, to shift the frequencies exhibited by natural rocking modes to prevent overlap with bearing defect frequencies, to accommodate differing effects resulting from varying the number of disks supported by a spindle, variations in the rotational speed of a spindle and various types of bearings, and to avoid and/or dampen the affect of vibration overall. One way is to modify the internal construction of the spindle to change the stiffness and thereby alter the rocking mode resonance to a non-interacting frequency. U.S. Pat. No. 5,483,397 to Gifford et al. discloses first and second viscoelastic dampers effective in attenuating vibrations during operation of a disk drive for improved disk drive performance. The first viscoelastic damper is inserted on top of the spindle shaft and in contact with the top cover of the disk enclosure. The damper has a layer of viscoelastic material fixed to one side of a washer which is effective to attach the washer to the top cover of the disk enclosure. The washer also has an opening at a raised central region to provide access for the attachment of the top cover and the shaft. The second viscoelastic damper is positioned outside the disk enclosure in contact with the bottom baseplate. It is constructed of a polyester layer disposed between first and second viscoelastic layers.
Further, low acoustic noise is an increasingly important performance advantage in the application of hard disk drives. For example, hard disk drives designed to operate in personal computers are used in relatively quiet environments. It has been found that a major source of acoustic noise is the excitation of the spindle-disk assembly during normal operation.
This prior art has several disadvantages. First, an additional mechanical component must be added to the disk drive assembly to attenuate vibrations during operation of the disk drive. This adds cost to the manufacturing of the disk drive. Second, the additional mechanical component decreases system reliability as there exists another component subject to mis-assembly or failure. Third, this prior art can increase head offsets (relative alignments of heads and disks down the spindle) over time and temperature extremes due to spindle movement because the spindle is not rigidly attached to the cover.
The present invention allows for tuning the structural stiffness of a spindle in a disk drive, to avoid and/or dampen rocking motion of the spindle motor.
A preferred embodiment containing principles of the present invention allows for tuning the structural stiffness of a spindle in a disk drive to reduce vibration, including the rocking mode resonance of a disk drive spindle either alone or when excited or enhanced by a complementary bearing defect frequency. Modification to the disk drive cover in the area of the spindle, in accordance with the principles of the present invention, will reduce the amplitude and/or alter the frequency of the natural rocking resonance mode of the spindle sufficiently either to prevent a previously interacting spindle to no longer interact at a particular bearing defect frequency or to dampen the amplitude of the rocking resonant mode, the bearing defect and overlapping combinative effects of both to reduce the effects of vibrations. This is accomplished by altering the stiffness or spring rate of the disk drive cover or by adding damping. With reductions of the in-plane stiffness of the disk drive cover, substantial reductions or shifting of rocking mode frequency may be accomplished, and vibration is eliminated or substantially reduced.
One embodiment consists of a series of slots in the top cover, extending radially outwardly from the area of the cover to which the spindle attaches. The design allows increased lateral motion of the spindle shaft by facilitating movement in the plane of the cover. The slots change the top cover from a homogenous stiff in-plane material, to a series of relatively weaker beams which flex yet contain no directional stiffness dependency. By modifying the length, spacing, width, and configuration of the slots, varying amounts of stiffness reductions are available. In this manner, the cover can tune the spindle resonance to avoid interaction with the bearing defect frequency, or to dampen any vibrations resulting from the rocking resonance mode or bearing defects, alone or combined.
In a second embodiment, circumferential slots are overlaid by a constrained-layer damper which sits in a pocket formed by a raised portion around the perimeter of the top cover. This acts to reduce the motion of the spindle shaft by absorbing energy. Consequently, the combination of a cover having slots allowing for more spindle shaft motion and the constrained-layer damper, changes the spring rate of the top cover and adds further damping to the rotating spindle-disk assembly. Thus, this embodiment is useful in avoiding an undesirable interaction with the bearing defect frequency and in further damping any interaction that may occur. The implementation of this embodiment has been shown to have attained greater than a 50 percent reduction in peak amplitude associated with rocking mode resonance significantly reducing position error.
A third embodiment consists of a series of spiral slots in the top cover, extending outward from the area of the cover to which the spindle attaches. Like the prior embodiments, the design alters the top cover from a homogenous stiff in-place material, to a series of relatively weaker beams which flex more easily. In this manner, the natural rocking mode resonance can be altered or shifted away from the bearing defect frequency to avoid overlap and negative interaction. While a damping material may also be incorporated, the damping effects are not as pronounced as in the other described embodiments.
It is an object of the present invention to reduce the deleterious effects of vibration and acoustic noise in a disk drive.
It is an object of the present invention to alter or reduce rocking mode resonance and acoustic noise by changing the natural frequency of the spindle.
It is an object of the present invention to eliminate the interaction between bearing defect frequency and rocking mode resonance.
It is another object of the invention to provide a disk drive with increased spindle resonance design flexibility by improving the adjustability of the stiffness of the disk drive housing.
It is another object of the present invention to provide a flexible top cover which reduces spindle vibration.
It is another object of the invention to dampen vibrations in a disk drive.
It is a further object of the invention to provide a disk drive that reduces parts counts and costs.
It is another object of the invention to improve disk drive reliability by eliminating mechanical components, reducing servo system failure resulting in write-to-write and write-to-read track misregistrations, and decreasing position error between the recording head and data tracks resulting in reduced track following capability.
These and other objects of this invention will become apparent from the following drawings and description.